Time Slicing Method

Tertiary storage devices provide huge storage capacity at low cost. Multimedia objects stored on the tertiary storage devices are accessed with high latency. Despite the high access latency, some tertiary storage devices are able to deliver data at high throughput. The time slicing method is designed to reduce the start-up latency in accessing multimedia objects from tertiary storage devices. The start-up latency is lowered by reducing the amount of data being migrated in stage one of the staging method being described in the last chapter. In order to support the time-slicing method, the tertiary storage devices should have the ability to deliver data at high throughput. The tertiary storage devices that cannot deliver data at sufficiently high throughput; the start-up latency cannot be reduced.

Tertiary Storage Devices

The main objective of the tertiary storage level is to provide huge storage capacity at low cost. Several types of storage devices are available to be used at the tertiary storage level in Hierarchical Storage Systems (HSS). They include: • Magnetic tapes • Optical disks • Optical tapes These storage devices are composed of fixed storage drives and removable media units. The storage drives are fixed to the computer system. The removable media unit can be removed from the drives so that the storage capacity can be expanded with more media units. When data on a media are accessed, the media unit is accessed from their normal location. One of the storage drives on the computer system is chosen. If there is a media unit in the storage drive, the old media unit is unloaded and ejected. The new media unit is then loaded to the drive. Each type of storage drive may handle the storage drives and media units differently. The magnetic tapes are described below in the next section. Then, the optical tapes are presented. Afterwards, the optical disks are briefly described before this chapter is summarized.

Segmented Pipelining

The robotic tape library and optical jukebox provide huge and cheap capacity for the storage of multimedia objects. The stored objects may be retrieved using staging, time slicing, or pipelining. The staging method retrieves the whole objects to the staging buffers prior to consumption at the cost of high start-up latency. The time slice method reduces the start-up latency at the cost of heavy switching overheads. The pipelining methods aim at minimizing the start-up latency. In the normal pipelining method, the sizes of the slices are minimized to maximize the overlapping between the displaying time and the retrieval time of the slices. In space efficient pipelining methods, the buffer size in accessing the slices is minimized. We have already described the normal pipelining and the space efficient pipelining methods in the two previous chapters. The segmented pipelining method to reduce the latency in serving interactive requests is presented in this chapter. Multimedia objects are usually displayed from the beginning to the end in video on demand systems. Interactive video-on-demand systems support VCR-like functions, including fast forward, rewind, pause, and resume functions. Large video systems store many objects. The video systems should allow some searching to allow users find the desired objects. When searching is required, the video-on-demand system would need to provide browsing, jump, keyword, and content based searching. Unless the staging method is used, the multimedia storage system cannot support any VCR-like operations. The segmented pipelining method is designed to provide efficient retrieval of multimedia objects with supporting of previews. In this chapter, the segmented pipelining method is first described in the next section. The performance of the segmented pipelining method is then described and analyzed before this chapter is summarized.

Staging Methods

When data are stored in the tertiary storage devices, the tape drives shall read them from the tapes using the input/output (I/O) operations. Due to the long delay in exchanging tapes, it is inconvenient to exchange a tape for each read/write access operation. Thus, the entire object or file is accessed from the tape drives well before they are being used (Federighi & Rowe, 1994; Kienzle, 1995; Pang, 1997). These accessed objects are temporarily stored in the magnetic hard disks as secondary storage level.

Statistical Placement on Hierarchical Storage Systems

We have described the contiguous placement in the previous chapter and the statistical strategy to place objects on disks in Chapter IV. In this chapter, we describe the statistical strategy to place them on hierarchical storage systems. The objective of the data placement methods is to minimize the time to access object from the hierarchical storage system. The statistical strategy changes the statistical time to access objects so that the mean access time is optimal. The objective of the frequency based placement method is to differentiate objects according to their access frequencies. The objects that are more frequently accessed are placed in the more convenient locations. The objects that are less frequently accessed are placed in the less convenient locations. We will describe the frequency based placement method in the next section. Afterwards, we will analyze its performance. Last, we summarize this chapter.

Data Compression Techniques and Standards

In the previous chapter, we see that the performance of a storage system depends on the amount of data being retrieved. The size of multimedia objects are however very large in size. Thus, the performance of the storage system can be enhanced if the object sizes are reduced. Therefore, multimedia objects are always compressed when they are stored. In addition, the performance of most subsystems depends on the amount of data being processed. Since multimedia objects are large in size, their accessing times are long. Thus, multimedia objects are always kept in their compressed form when they are being stored, retrieved, and processed. We shall describe the commonly used compression techniques and compression standards in this chapter. We first describe the general compression model in the next section. Then, we explain the techniques in compressing textual data. This is followed by the image compression techniques. In particular, we shall explain the JPEG2000 compression with details. Lastly, we explain the MPEG2 video compression standard. These compression techniques are helpful to understand the multimedia data being stored and retrieved.

Cooperative Web Caching

Most clients are placed behind the proxy servers on the Internet. Proxy servers have the disk cache space, network bandwidth, and availability to cache part of the objects for clients. In addition, the number of proxy servers can be increased or decreased dynamically according to the anticipated server workload, making them good candidates to alleviate the bottleneck problem. We have described in the last two chapters how the caching methods provide better performance for continuous request streams in individual proxy servers. In this chapter, we show how the proxy servers may work together to improve the overall performance in delivering objects. At present, large multimedia objects are not cached or only partially cached in current proxy servers mainly for two reasons. First, the owner of the multimedia objects needs to ensure security and control of access of the objects before they are willing to let any proxy servers cache their objects. Thus, any new methods need to allow the content owner have complete control over the objects’ security. Second, the owner of the proxy server wishes to have full autonomy control over its own cache content so that the proxy server may maximize the cache efficiency for its own clients.

Normal Pipelining

Multimedia objects can be stored on tertiary storage devices to provide large storage capacity at low cost. The staging method retrieves the whole objects to the staging buffers prior to consumption. Thus, the start-up latency is high. The time slice method being described in the last chapter reduces the start-up latency only when the tertiary storage bandwidth is higher than double of the displaying data rate of the object. However, if the tertiary storage bandwidth is below double of the data consumption rate of the object, then we can only stage the object prior to using it. The pipelining methods aim at minimizing the start-up latency when the tertiary storage bandwidth is not higher than the data consumption rate of the objects. The pipelining methods are used to reduce the start-up latency and staging buffer size. In the normal pipelining method, the sizes of the slices are minimized to maximize the overlapping between the displaying time and the retrieval time of the slices. In the space efficient pipelining methods, the buffer size in accessing the slices is minimized. In the segmented pipelining method, the latency in serving interactive requests is reduced. The normal pipelining method is described in this chapter. The space efficient pipelining method and the segmented pipelining method are presented in the following two chapters. We shall describe the objective of the normal pipelining method. Then, the bounds on the sizes of the slices are shown. After that, the start-up latency and the minimum size of the first slice are shown. The reduction in the startup latency using the normal pipelining method is presented.

Contiguous Placement on Hierarchical Storage Systems

The contiguous placement is the most common method to place traditional data files on tertiary storage devices. The storage space in the media units is checked. The data file is stored on a media unit with enough space to store the data file. When tertiary storage devices are used to store multimedia objects, the objects are stored and retrieved similar to traditional data files. Since the main application of the tertiary storage devices is to back up multimedia objects from computers, the objectives of the contiguous method are: 1. supporting back up of multimedia objects efficiently, and 2. reducing the number of separate media units that are used to store an object. We will describe in the next sections the simple contiguous placement method. Afterwards, the log structured placement method is explained before we summarize this chapter.

Statistical Placement on Disks

The access pattern on each multimedia object can have very different characteristics. Some multimedia objects are more popular and they are more frequently accessed by more users. The user may concern the average access time on the objects. Thus, the storage systems can make use of the popularity of multimedia objects to optimize the average access time. Some objects need to be accessed at a higher data rate than the other objects. The users may concern the continuity of these objects. The storage systems may store the high data rate objects at the locations where data transfer rates are higher. The statistical placement methods place the multimedia objects according to the characteristics of their access patterns. We shall describe the frequency based placement or popularity based placement method which optimizes the mean access time as the performance metric in the next section. After that, we shall describe the bandwidth based placement which uses the object continuity as the performance metrics. When two placement methods are compared on the same storage system, each one of the methods may show better performance according to different metrics. System builder may choose the appropriate method according to the method that shows better performance in the preferred metrics. Thus, both placement methods have their significance.